bisect_plane not clearing in python API
Set the normal
The default values of cutting plane coordinate and normal are (0, 0, 0).  A zero length normal is invalid and quite likely does nothing.
Test script
Here is a minimal working example of a script, adds a cube, runs the operator with plane coordinate at origin and its normal as the x axis. Recommend setting up a little script like below when asking questions
import bmesh
import bpy
from bpy import context

bpy.ops.mesh.primitive_cube_add()
ob = context.object
me = ob.data
bm = bmesh.new()
bm.from_mesh(me)


print('Nverts0',len(bm.verts), len([v for v in bm.verts if v.is_valid]))
bmesh.ops.bisect_plane(
    bm, 
    geom=bm.edges, 
    dist=0.0001, 
    plane_co=(0, 0, 0), 
    plane_no=(1, 0, 0), 
    use_snap_center=False, 
    clear_outer=True, 
    clear_inner=False,
    ) 
print('Nverts1',len(bm.verts), len([v for v in bm.verts if v.is_valid]))
bm.to_mesh(me)

Result
Nverts0 8 8
Nverts1 12 12

Effectively bisecting all x axis aligned edges, creating 4 ngons with 6 verts.  Nothing to clear as faces are not &quot;ripped&quot;.
Input geometry
For clear to work on a mesh that has faces, we are going to need to add the faces and verts to the geometry input of the operator.  Recommend with geom giving all geometry of a region of interest will make all options work as expected.
    geom=bm.verts[:] + bm.edges[:] + bm.faces[:], 

making this change an running again
Nverts0 8 8
Nverts1 8 8

Has bisected cube, same vert count one less face.  The &quot;outer&quot; is cleared, ie all resultant geometry on the input normal's side of cutting plane.

Return Dictionary
Bmesh operators return information in a dictionary.
ret = bmesh.ops.foo.bar(blah)

in the case of bisect plane it has members &quot;geom_cut&quot; the edges and verts of the cut and &quot;geom&quot;.
Some Examples of using bmesh bisect plane operator.
Python: Bisect mesh into n parts, without separate &#39;LOOSE&#39;
using bisect but still failing
Slice up terrain mesh into chunks
How to get part of a mesh with Python API, which requires breaking down some faces?
Cutting a mesh in half
How to extract side-view outline (e.g. top view) of a 3D object to 2D surface?
Transform cylinder to flat plane - cylinder maze map